When an electromechanical control apparatus is opened or closed while live, an electric arc occurs. Repeated arcing is disadvantageous in that it erodes contacts progressively, and it also modifies the surface composition of the contacts, thereby leading to changes in contact resistance and thus to the danger of heating which is undesirable from the safety point of view. In addition, and above all, the presence of such arcing on closure runs the danger of dynamically bonding or welding together the two contact elements.
The contact materials most frequently used in low tension electromechanical apparatuses are composite silver/metal oxide materials, written AgMeO.sub.n. Silver is an excellent conductor of electricity, does not oxidize in air, and possesses sufficient malleability to withstand without damage the numerous mechanical shocks to which it is exposed in such an environment. The metal oxide is mainly present for reducing the danger of bonding compared with using pure silver. In addition, these composite materials are eroded away by arcing at a slower rate than is pure silver.
Heretofore, most contacts for power control apparatuses have been made of a composite silver/cadmium oxide material (AgCdO). However, because of the toxicity of cadmium, and also in order to improve the quality of these materials, attempts have been made to replace cadmium by some other metal. Tin turns out to be suitable on several counts.
However, it turns out that AgSnO.sub.2 materials tend to increase heating in the apparatuses because of the increased contact resistance between two facing pellets.
It is therefore conventional to add another oxide, generally WO.sub.3 or MoO.sub.3 to the contact element in order to mitigate this drawback.
However, this addition is disadvantageous in that it complicates the manufacture of contacts and makes such manufacture more expensive.
The object of the present invention is to mitigate these drawbacks of the prior art and to propose a method of preparation which makes it possible to obtain a contact material of the Ag/SnO.sub.2 type having improved properties.
More precisely, in spite of the above-mentioned drawbacks related to using SnO.sub.2 on its own in a silver-based composite material, the Applicant has continued research on AgSnO.sub.2 composite material and has discovered that under certain conditions it is possible to obtain an AgSnO.sub.2 composite which is at least as satisfactory as an AgCdO composite with respect to mechanical and electrical behavior, and that this can be done without there being any need to add additional oxides such as WO.sub.3 or MoO.sub.3 in order to avoid arcing problems.